You can change your phone number for your Changi Account easily, via these stpes:
1. Click on "Account" on the navigation bar on Changi App
2. Select the tool icon on the top right of the page
3. Select "Profile"
4. Under "Contact Details", enter the relevant phone number
5. Follow the verification steps to verify your phone number on Changi Account
6. Click on "Pay" on the navigation bar on Changi App
7. You will be prompted to verify your phone number again for Changi App
Follow these steps and you are all set!